This Fascinating view of the earth is a global average of data recorded by the tongue twisting Sea-viewing Wide Field-of-view-Sensor (SeaWiFs) over the entirety of its 13 year operational history – 1996-2010.
During its lifespan this sensor on GeoEye’s OrbView 2 satellite measured worldwide levels of chlorophyll – the green pigment which allows plants to obtain energy from light. As NASA Earth Observatory explains, at sea the colours in the map represent areas where phytoplankton is most common, on land the the density of vegetation:
An NDVI of zero means no green plants and a high value (0.8 or 0.9) is a thick canopy of green leaves.
As to why monitoring phytoplankton levels is really important:
It’s the blooming and die-off of the phytoplankton that form the center of the oceanic food web…a direct indicator of the seas’ ability to support life.